This invention relates to the detection of corrosion of structural materials within a fluid flow path. More particularly, the invention relates to a method and apparatus for detecting corrosive contaminants in combustion gases in gas turbines. Even more particularly, the invention relates to a method and apparatus for detecting corrosive alkali metal contaminants in combustion gases in gas turbines.
Many structures, such as aircraft and power turbine assemblies, reactors and cracking columns in chemical plants and refineries, blast furnaces, and nuclear reactors, include structural components that are exposed to fluid flows containing potentially corrosive contaminants. For example, advanced gas turbines, designed for power generation, use metal alloys that are selected to provide maximum strength at the high temperatures encountered in the first stage buckets. As a result, such alloys are less resistant to hot corrosion than alloys operating at lower temperatures.
The presence of sodium and other alkali metals in fuel, air, water, or steam supplied to the combustion chambers of gas turbines can create a gaseous mixture that is corrosive to the high temperature alloys used in a hot gas path. Alkali metals may initially be present as salts in either the fuel or air ingested by a turbine assembly. Seawater, for example, may mix with fuel transported in tanker ships. Similarly, turbine assemblies sited on or near a coastline ingest air having a high salt content. Alkali metal salts, once ingested, may combine with sulfur in the fuel to form alkali metal sulfates that condense or impinge upon turbine components. Metals present within the alloys, such as nickel, cobalt or iron, may react with the molten alkali sulfates to form a corrosive liquid over a wide temperature range, resulting in the phenomena known as hot corrosion. Hot corrosion leads to a considerable reduction in the life of turbine components.
Techniques are presently available for detecting and measuring the concentrations of corrosion-causing species in turbine supply lines. The concentration of sodium in fuel oil, for example, can be measured by atomic absorption, inductively coupled plasma (ICP)/atomic emission, and mass spectroscopy. These analytical methods are designed for use under laboratory conditions and are not commonplace in industrial settings, such as those encountered at power generation facilities. In addition to the above-mentioned techniques, sodium ion selective electrodes can also be used to measure sodium concentrations in aqueous media, and ion chromatography has also been employed for alkali metal detection. None of these technologies has been applied to monitor the combustion gases in a gas turbine, and none provide a direct indication of the corrosivity of the gases.
Although sensors are currently being used to monitor sodium levels in water and other systems are being developed to measure sodium in fuels, it is impractical to measure the presence of sodium or other potentially corrosive contaminants in a fluid flow path, such as the air that is supplied to a gas turbine. Therefore, what is needed is a sensor capable of detecting the total amount of corrosive contaminants, such as sodium or other alkali metals, in a fluid flow path. What is also needed is a sensor capable of measuring the degree of corrosion of structural components exposed to such a fluid flow path.
A corrosion sensor for use within a fluid flow path includes a sensing element and circuitry coupled to the sensing element for detecting corrosion of the sensing element. Another embodiment of a corrosion sensor for use within a fluid flow path comprises a hermetic housing, a sensing element attached to an external portion of the housing for exposure to the fluid flow path, and circuitry disposed within said hermetic housing, which circuitry is coupled to the sensing element for detecting corrosion of the sensing element.
These and other aspects, advantages, and salient features of the invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.